Date: Sat, 26 Aug 95 08:21:41 -0700
From: Lynn Sebourn
To: billbeskimo.com
Subject: shock wave on wings
Dear Bill,
I was just looking at your photograph of the wing. It is correct that
this phenomenon is caused by a shock on the wing. All large passenger
aircraft travel very close to the speed of sound, (Mach 1). As the air
travels over the wing it must speed up over the top, therefore over the
top of the wing, the flow accelerates past Mach 1 and is actually
supersonic for a small distance over the wing. At some point, however it
must slow back down as it approaches the trailing edge of the wing. The
shock you see on the wing is where the flow suddenly is decelerated from
supersonic to subsonic flow.
The reason you can see the shock is because at the shock interface, the
air is compressed very rapidly. Therefore the density just downstream of
the shock is much higher than the density just upstream of the shock. If
you are familiar with optics, you have heard of the index of refraction.
Light bends as it travels through different substances such as glass,
water, and even air. This is why eyeglasses work and why things under
water appear to be in a different place than they really are. (Ever try
to catch a fish by hand?). In air, the amount of refraction depends on
the density of the air. Since there is a sudden change of density near
the shock, as light travels through the shock, it is bent different
angles, depending on whether it passes in front of or behind the shock.
The result is a series of light and dark bands which allows you to "see"
the shock.
This same principle is put to use in wind tunnels to aid in seeing the
flow around wings and aircraft.
Just thought you might like to know.
Lynn Sebourn
Aerospace Engineer